Mirror Lamps on x axis
You can do it manually with a duplicate lamp:


Space, Snap Cursor to Center, Enter
Set the pivot center in the toolbar on the bottom to 3D Cursor
Select your lamp, S,X,-,1, Enter
Done.


If the lamp is changing position more often, duplicate it and add an object constraint:


Select the dupli lamp
Properties, Constraints, Add Object Constraint, Copy Location
Check Invert for the axis you want to mirror.
For directional lamps, also try Copy Rotation